Preliminary modeling of a plasma gasification system for marine transportation

Cruise ships are complex systems that strongly resemble municipal communities, as they must manage a significant and diversified amount of waste streams, often while far away from the shore. Different waste management techniques are employed on vessels, such as storage and off-loading, discharge at sea (if possible) and incineration. The scope of this study was to develop a model for a plasma gasification system and to simulate the treatment process of different feedstock compositions, based on the waste date obtained from the literature. A parallel model for an RF plasma torch as part of plasma generation system, i.e. plasma generator was also developed, to assess quantities which are needed by the gasification model. The two models were used to investigate the possibility for the system to operate in conditions of electricity net gain, to assess the efficiency of the gasification process through the cold gas efficiency parameter, but also the emissions generated, the heat that can be recovered, the sorbent consumption of the gas cleaning system. In the first section, the environmental issues and the current waste management techniques were investigated, with some references to international regulations. In addition, the different waste-to-energy approaches were described, focusing on the advantages and disadvantages of plasma gasification; subsequently, the modeling phase was carried out, starting from the development of the plasma generator model. Finally, the overall gasification system, comprising of waste pre-treatment, gasification, syngas cleaning, energy conversion, heat recovery sub-models was modeled. Four different feedstock composition scenarios were investigated. The results of the simulations have shown that the system developed is able to treat variable compositions of the waste feedstock, in a mixture of solid wastes, plastics and sewage sludge, while also maintaining a positive balance in terms of electric energy in a suitable range of mass flow rates of feedstock. The heat recovered is reduced for higher contents of sewage sludge, up to -12% in scenario 2 (integration of sewage sludge only) compared to the scenario with only solid wastes. The introduction of plastic allows for a higher heat recovered, up to +23.6% in scenario 4 (integration of sewage sludge and plastic). The cold gas efficiency is increased for higher plastic and sewage sludge contents, up to a maximum value of 58.65%. Emissions are within the limits prescribed by the MARPOL Annex VI regulation (6 gSOx/kWh and 7.84gNOx/kWh) and strongly linked to sewage sludge content. The results of the torch modeling provide expected results in terms of temperatures and air mass flow rates. This preliminary study suggests potential feasibility of such systems. However, in future research works, the models will need validation through experimental setups. Improvements to the model will also be needed, to increase the complexity of the physics involved and to investigate some additional plasma gasification issues, like tars formation, clogging and dioxins formation.